This invention relates generally to optical communication, and more particularly the invention relates to a method of treating silica optical fibers and other silica based material using heat and a poling electric field to induce optical non-linearities therein as necessary for optical modulation and switching.
Phase modulators and switches operating in the gigahertz to multigigahertz range are key devices needed for optical communication systems, local area networks, and fiber sensors, in particular in the fiber gyroscope. To date, low voltage phase modulation and switching at reasonably high frequency ( greater than 1 MHz) is not available in all-fiber component, of any form. It is typically provided by a LiNbO3 integrated optic (IO) channel waveguide device, which relies on the electro-optic properties of LiNbO3 to modulate the phase of the optical signal via an externally applied electric field. These IO devices, however, introduce a relatively high coupling loss, at least 1 dB, when inserted in an optical fiber circuit. The process of physically coupling the IO device to the fiber, called pigtailing, is also costly and difficult. Replacing these IO channel waveguide modulators by an all-fiber modulator would eliminate the high cost and technical difficulties by providing a device which can be directly spliced to the circuit fiber. Splicing would also eliminate altogether mechanical instabilities and undesirable reflection losses which typically occur at a fiber/IO interface.
In the communication area, the loss constraints are even greater. A large portion of the fiber-optical market is projected to be multi-user systems involving hundreds of serial switches, with exceedingly low overall loss, typically under 0.1 dB. Such a stringent requirement again essentially rules out IO components. These considerations practically dictate that these devices must be made in a glass-based fiber. Many other applications will also benefit from the low transmission loss of fiber-based devices in the far IR and UV, and other electro-optic devices, electro-optic fiber modulators and switches can thus be used over a broad range of wavelengths and powers.
An object of this invention is a new process which will induce a large electro-optic coefficient in glass-based materials, including, but not limited to, standard optical fibers and IO waveguides made in fused silica, and thus make it possible to fabricate low-voltage electro-optic phase and amplitude modulators and switches in a single-mode fiber. Like their IO counterpart, these devices will be driven directly by a low voltage, low current electrical signal.
Briefly, a silica optical fiber or other glass based material is placed in an oven within an enclosure which is preferably a vacuum chamber. Temperature in the oven is ramped from room temperature to a maximum value T1, which is on the order of 450xc2x0 C. or higher. The temperature is ramped at a rate which will not physically damage the glass material. An electric field is applied to the silica material either during the temperature ramping or after the maximum temperature is reached. The field should be as high as possible, on the order of 800 V/xcexcm or higher, and/or the voltage should be as high as possible (10 kV or higher), without exceeding the dielectric breakdown of the material. The temperature and electric field are maintained for a period of time (poling time) which is in the range of seconds to several tens of minutes. The temperature is then ramped down at a rate slow enough to allow the material to be in constant poling equilibrium. The voltage is switched off when the temperature of the sample has dropped to a low enough value, ideally room temperature (typically 20xc2x0 C.) although higher temperatures (such as 50xc2x0 C.) may be also suitable.
The invention including processing alternatives will be more readily apparent from the following description and appended claims when taken with the drawings.